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Biology 2430 Lecture Notes
Chapter 21: Genetic Basis of Cancer
- Outline
- Relationship of cell cycle to cancer
- Molecular control of cell cycle
- Regulation of cell division in normal cells
- Cancers are genetic diseases
- Genes and cancer
- Oncogenes
- Tumor suppressor genes
- p53 tumor suppresssor gene
- Mutator genes
- Telomere shortening, telomerase, and human cancer
- The multistep nature of cancer
- Chemical and radiation as carcinogens
- Relationship of cell cycle to cancer
During development of multicellular organisms, genetically preprogrammed cell division and differentiation results in specific tissues and organs. In adults, many cells also are capable of proliferating in a controlled way in order to replace cells normally lost during normal processes in the bofdy or those lost to injury. However, sometimes, dividing and differentiating cells deviate from their normal genetic program, leading
to a proliferative mass of clonal cells called a tumor. A cell is said to be transformed when it loses the ability to remain constrained in its growth properties. When these transformed cells stay together in a single mass, the tumor is begnin (more easily treated by surgery). In malignant Tumors, cells detach from primary tumor and invade surrounding tissue. These are identifyed as cancer. During metastasis, cells from a malignant tumor spread via the circulatory or lymphatic system and set up new tumors in other locations in the body. Death results because of damage to critical organs, secondary infections, metabolic problems, or hemorrhage. In this chapter we focus on the genetic basis cancer.
In Canada, almost half of all adults die of cancer; some cancers more frequent
than others; some more treatable than others. Different types of cancer may have different genetic basis, and result
from different cell lines which accumulate mutations in cancer-causing
genes.
- Cancer results from uncontrolled cell
proliferation. Control of overall population size based
on regulated interplay between those processes that lead to increase
in cell numbers (progression through cell
cycle), and those that lead to a decrease in cell numbers
(apoptosis).
- Cell cycle and apoptosis programs regulated by both positive and negative control mechanisms.
- Unregulated cell division easily seen in tissue culture. Normal fibroblasts grow in culture flasks only until they touch each other and form a monolayer. In this type of regulated growth, cells can communicate with each other and stop cell division. This is called contact inhibition.
- Transformed cells do not show contact inhibition, pile up onto one another and grow into many layers in tissue-culture flasks.
Relationship of cell cycle to cancer
- Tissues arise by cell division and differentiation, in which progeny cells express tissue-specific genes. As cells differentiate, they also lose their ability to proliferate. Terminally differentiated cells do not divide. They are replaced by cell division from unpecialized cells, called stem cells, which are capable of self-renewal. Malignant cells can replicate, but fail to express the normal genetic programs completely.
- Molecular control of cell cycle
- Cell cycle is controlled by an elaborate system of checks and balances. Checkpoints refer to contol points at different stages of the cell cycle in which cell cycle can be arrested if there damage to genome or cell cycle machinery (Fig 22.2). These elaborate control processes are necessary to prevent unprogramed cell proloferation (ie cancer).
- Much of the information of how cells regulate cell cycle have come
from studies with Yeast.
- Genome completely sequenced and all genes are known.
- Easy to genetically manipulate and carry out experiments.
- All genes for cyclins and CDKs are known. These proteins regulate
progression through the cell cycle.
- Mutants available that are blocked at various stages of cell cycle. Many of these genes have been cloned. Many of these have homologs in humans with similar functions. Mutant versions of many of these genes cause cancer.
- Genome completely sequenced and all genes are known.
- Different Cyclin and cyclin-dependent protein kinases accumulate at
different phases of the cell cycle and regulate the progression from
one phase to another.
- CDK Targets: transcription factors
that promote transcription of certain genes whose products are required
for the next stage of the cell cycle (including the next set of cyclins
and CDKs).
- Target proteins for CDK phosphorylation determined by associated cyclin.
- Phosphatases rapidly dephosphorylate
target proteins in absence of cyclin-CDK complex.
- Example: Levels of CdK2-cyclin
A builds to critical levels at end of G1. Phosphorylation of Rb by cyclin-cdk complex causes it to disassociate from E2F transcription
factor. Unbound E2F binds regulatory genes for enzymes necessary for
DNA synthesis, allowing transition from G1 to S phase.
- Switch from G1 to Sphase also under Negative Intracellular control,
by p53.
- Progression from G1 to S phase inhibited by DNA damage.
- Progression from G1 to S phase inhibited by DNA damage.
- p53 levels increase after DNA damage and activate p21.
Elevated levels of p21 suppress kinase activity of cyclin-cdk
complex, causing E2F to remain bound in its inactive state. Therefore,
S-phase does not begin.
- Advantageous strategy to reduce mutation rate, and maintain genetic identity within clonal aggregate.
- Regulation of cell division in normal cells
- In normal cells, cell division is controlled by extracellular (e.g. growth factors and inhibitors, hormones, cytokines) signals as well as intracellular signals. Cells only divide when there a proper balance of stimulatory and inbibitory signals from outside the cell.
- Growth factors and growth-inhibitory factors exert their affect on normal cells by binding to receptors on the cell membrane, initiating a change in the internal part of the receptor which relays the signal through a series of proteins, eventually activating transcription factors which turn on specific genes whose products either stimulate or inhibit cell division (Fig 22.3).
Cancers are genetic diseases
- Evidence that cancers are genetic diseases:
- Some cancers run in families (e.g. retinoblastoma is hereditary) (most cancers are nonhereditary)
- Some viruses cause cancer. Some viral genes when expressed disrupt normal cell controls.
- Descendants of cancerous cells are all cancerous.
- Cancers increase upon exposure to mutagenic events (chemicals, radiation).
- Certain chromosomal mutations are associated with some cancers.
Genes and cancer
- Genes that cause cancer are divided into three classes: Oncogenes, tumor supprressor genes and mutator genes.
- Oncogenes
- Oncogenes are mutated versions of normal genes that stimulate cell proliferation. These "normal" genes are called proto-oncogenes.
- Oncogenes are dominant: cause cancer when present in a single copy (allele).
- Tumor viruses cause cells to proliferate uncontrolably. First discovered in Rous-Sarcoma virus, which causes cancer in
chickens (and humans). Many cancer-causing viruses have been identified
since then. Two types: RNA tumor viruses (retroviruses) and DNA tumor viruses.
- RNA tumor viruses (Fig 22.4) :
- contain viral oncogenes (v-oncs) that gives them the ability to transform infected cells (e.g. RSV Fig 22.5). These oncogenes are derived from the host cell, and are celled c-oncs. v-oncs differ from c-oncs in that they are shorter and lack introns (Fig 22.7).
- different retrovirues have different oncogenes (Table 22.1).
- most oncogenic virues cannot replicate because they lack the full set of life cycle genes.
- Nononcogenic viruses have a complete set of life cycle genes and lack v-oncs, thus do not cause transformation in cells they infect (Fig 22.6).
- DNA tumor viruses
- Do not carry oncogenes; transformation of cells occurs through the activity of viral genes that activate progression through the cell cycle.
- E.g. papillomaviruses (cause venereal warts and cervical cancer). Vaccination against the virus protects against cervical cancer.
- RNA tumor viruses (Fig 22.4) :
- Three kinds of genetic changes can make proto-oncogene into an
oncogene :
- 1. hyperactivation (i.e. gain of function mutation)(normal
protein levels).
- 2. gene duplication (excess protein levels)
- 3. translocation bring proto-oncogene under influence of new
promoter leading to increase transcription rates (excess protein
levels).
- 1. hyperactivation (i.e. gain of function mutation)(normal
protein levels).
- Proto-oncogenes normally encode for components of signal transduction pathways that stimulate cell division. Any mutation in any of these components has the potential of turning it into an oncogene (e.g. ras).
- Over 100 different oncogenes have been discovered (Table 22.2). These genes encode proteins often involved in signal
transduction pathways regulating various aspects of cell cycle.
Types of oncogenes include:
- transcription factors
- intracellular transducers
- mitogens (signaling molecules that promote cell division,
e.g. growth factors).
- mitogen receptors
- apoptosis inhibitors
- transcription factors
- Mitogens are molecules that stimulate cells to proliferate.
- Mitogen activated pathways exerts positive control on cell
cycle. One of these pathways involves ligand binding to a Receptor Tyrosine
Kinase (RTK) (Fig 22.8)
- Pathway for RTK
signalling
- growth factor (mitogen) binds receptor. Causes dimerization leading
to activation of kinase activity inside cell.
- kinase activity causes Ras to switch from inactive (GTP
bound) to active (GTP bound) state. Ras is a G-protein common
in signal transduction pathways.
- active Ras leads to activation of MAP
Kinase Cascade, followed by activation of transcription
factors.
- transcription factors bind cis acting regions, leading to expression
of genes involved in cell division.
- Mitogen activated pathway can be integrated with other sensory
information in different signal transduction pathways.
- growth factor (mitogen) binds receptor. Causes dimerization leading
to activation of kinase activity inside cell.
- Mutation of certain genes in RTK signaling pathway convert them into
oncogenes.
- Defective ras always
bound to GTP, therefore constitutively transduces signal for cell
to divide.
- Mutation in RTK also results in oncogene (v-erbB); defective RTKs dimerize without presence of ligand, activating their intracellular kinase activity. Like ras oncogene, defective RTK causes constitutive signalling leading to cell proliferation.
- Defective ras always
bound to GTP, therefore constitutively transduces signal for cell
to divide.
- Tumor suppressor genes
- Discovered when normal cells were fused with cancer cells, resulting in hybrid cells that had normal gowth poattern. It was hypothesized that normal cells containd gene products capable of suppressing uncontrolled cell proliferation.
- Tumor-suppressor genes are recessive. i.e. need both copies to
be defective.
- Tumor-suppressor genes normally encode for components of signal
transduction pathways that are involved in inhibition of cell division
or the promotion of apoptosis. Any of these components, if defective
can prevent inhibition and therefore lead to proliferative growth (Table 22.3).
- Retinoblastoma Tumor suppressor gene, RB
- retinoblastoma develops from birth to the age of 4 years; most common eye tumor in children (Fig 22.9). Two forms of disease are explained by the two-hit-model (Fig 22.10):
- sporatic retinoblastoma
- arises spontaneopusly; no family history
- usually tumors are unilateral, and occurs later in life.
- both genes must mutate
- hereditary retinoblastoma
- develop multiple eye tumors in both eyes
- tumors develop at much earlier age.
- one mutated rb allele is inherited. Individual only needs one mutation for both to be inactivated.
- The human RB tumor suppressor gene is located on chromosome 13, is 180 kb long, encodes 4.7 kb mRNA encoding a 928-amino acid nuclear phosphoprotein, pRB.
- pRB normally binds and inactivates the transcription factor E2F. When pRB is dephosphorylated, E2F activity in inhibited (Fig 22.11). When pRB is phosphrylated, it releases and activates E2F, which then turns on transcription for the synthesis of cyclin A. A Cdk-2-cyclin A complex activates DNA replication.
- sporatic retinoblastoma
- retinoblastoma develops from birth to the age of 4 years; most common eye tumor in children (Fig 22.9). Two forms of disease are explained by the two-hit-model (Fig 22.10):
- TP53 tumor suppresssor gene
- When both alleles mutated, TP 53 involved in half of all cancers.
- Human TP53 gene located on chromosome 17, and encodes a 393-aa p53 tumor suppressor protein. p53 is a transcription factor that is regulated by phosphorylation and by its inteaction with Mdm2 (also a phosphoprotein).
- In normal cells, p53 and Mdm2 are unphosphorylated, making them associate. Mdm2 leads to degradation of p53, therefore p53 levels are low.
- When there's DNA damage, p53 initiates events that lead to arrest at G1 and stimulates expression of genes required for DNA repair. (Fig 22.12) .
- p53 also plays a role in apoptosis (see below)
- If DNA is beyond repair, p53 does not induce DNA repair genes or WAF11. Instead, it activates BAX gene. BAX protein blocks of BCL-2 protein, whose function is to act as
suppressor of apoptosis by sequestering free cytochrome c molecules.
- In follicular lymphoma, a translocation has juxtaposed the BCL-2 gene in the vacinity of an enhancer that usually drives expression of an antibody gene. Over-expression of Bcl-2 gene leads to abnormal halting of apoptotic signals leading to cell proliferation (dependent on other genetic changes).
- Without active BCL-2 repressor, the apoptotic pathway is activated.
- TP53 nockout mice are viable but show high frequency of cancers, supporting its role in tumor suppression and maintenance of genetic stability in cell.
- If DNA is beyond repair, p53 does not induce DNA repair genes or WAF11. Instead, it activates BAX gene. BAX protein blocks of BCL-2 protein, whose function is to act as
suppressor of apoptosis by sequestering free cytochrome c molecules.
- Breast Cancer Tumor Suppressor Genes
- Breast cancer represents more than 31% of cancers in women. About 5% of these are hereditary. The hereditary form of breast cancer occurs earlier and often affects both breasts.
- BRCA1 and BRCA2 are two cancers genes implicated inhereditary breast cancer (as well as ovarian cancer). Their precise function is unknbown, but are hypothesized to be tumor suppressor genes.
- Discovered when normal cells were fused with cancer cells, resulting in hybrid cells that had normal gowth poattern. It was hypothesized that normal cells containd gene products capable of suppressing uncontrolled cell proliferation.
- Mutator genes
- DNA repair genes: type of tumor suppressor genes; recessive (although hereditary form can appear dominant); when both copies mutated, cells unable to repair damaged DNA, leading to increased mutation rates. Many of these mutations will lead to formation of oncogenes and defective tumor-suppressor genes.
- Example: Hereditary nonpolyposis colon cancer (HNPCC).
- Autosomal dominant disease associated with early colorectal cancers. Do not form begnin tumors or polyps.
- Four human genes implicated : hMSH2, hMLH1, hPMS1, and hPMS2. All have homologues in E. coli and yeast which function in Misatch repair).
Apoptosis
- Proper growth and development in multicellular organisms requires
the ability of some cells to carry out a genetic suicide program, known
as apoptosis, or programed cell death. Phagocytic cells
remove apoptotic cells from tisssues in a clean and tidy manner. Apoptosis
is different from another form of cell death called necrosis (uncontrolled cell death leads to cell lysis, inflammatory responses,
and serious health consequences).
- Apoptosis follows characteristic, genetically programmed series of
events that ultimately leads to cell death.
- Apoptosis invoves the activation of caspases.
- caspases formed as zymogens (inactive enzymes). Cleavage result in active caspase capable of degrading target proteins. These signals ultimately lead to DNA fragmentation, loss of normal cell shape, breakdown of organelles, fragmentation of cell. These are eliminated by macrophage.
- Apoptosis is under Positive Extracellular control.
- Self-destruction signal in lymphocytes activated through the Fas
system.
- Fas ligand (FasL); Fas receptor; Apaf (triggers caspase activation cascade).
- Self-destruction signal in lymphocytes activated through the Fas
system.
Telomere shortening, telomerase, and human cancer
- With the exception of germ line cells and stem cells, normal cells undergo replicative senescence because their telomeres shorten with every round of DNA replication. TRHese cells have little or no telomrase activity. Cancer cells are immortal because the telomerase enzyme is reactivated as a secondary event, enabling the cells to maintain telomere length and stabilize their chromosomes.
- In some animal tumors, inhibitionof telomerase activity leads to telomere shortening and replicative senescence.
- Telomerase makes a good target in cancer treatment.
The multistep nature of cancer
- Need several independent mutations at different loci for cell to
become cancerous (Fig 22.13).
- e.g. Colon cancer : involves sequenctial loss of apc gene, followed by mutation
to ras, and finally loss of p53 gene. Loss of p53 gene important because cell will be unable to induce apoptosis, nor
to be able to fix DNA damage prior to cell division. This results in
high mutation rate, with concomitant severity of disease. Mutations
in key genes allow cancer cells to proliferate and digest through basil
lamina to invade other locations.
- For cells to become cancerous, multiple genes must be mutated to stimulate progression through cell cycle, remove inhibitory control over cell cycle, and eliminate apoptosis.
End of chapter questions: 1, 2, 4, 6, 7, 8, 10, 11, 13, 15, 16, 17, 19-24, 27
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